When an engine is operating on diesel fuel, the power output of the engine is governed by smoke emissions in the exhaust. To increase the power of the engine, the amount of diesel fuel burnt in each engine cycle needs to be increased, but this creates unacceptable dense black exhaust emissions, as the relative proportion of diesel fuel to air is increased. Typically, the maximum amount of diesel fuel which can be introduced into the engine before the exhaust emission becomes unacceptable is only about 70% of the maximum which can undergo complete combustion in the engine with the corresponding amount of air which is induced into the engine. Hence the power output of the engine is limited.
When an engine is operating on gaseous fuel, the engine does not suffer from the same exhaust problem, and hence up to 100% of the maximum proportion of gaseous fuel to air which can be completely combusted in the corresponding amount of air, can be introduced into the engine, so that the engine can develop maximum power.
To overcome the exhaust emission problems with conventional diesel engines, a turbocharging or supercharging system may be used. Conventionally, in a turbocharged arrangement, a turbine wheel is operated by the exhaust gasses of the engine, and the turbine wheel in turn rotates an air compression wheel. In a supercharged arrangement, such an air compression wheel is powered by alternative power means such as a direct mechanical coupling between the air compression wheel and an output shaft of the engine, or an auxiliary power means. In each case, the air is introduced into the engine under compression (boost pressure) so that a greater quantity of air is present. Thus more diesel fuel can be introduced into the engine in each cycle of operation, whilst the proportion of diesel fuel to air need not exceed the 70% maximum permitted before the exhaust emission becomes unacceptable. Because more diesel fuel can be thus introduced, the power output of the engine is correspondingly increased.
However, a turbocharging or supercharging system in an engine operating on gaseous fuel cannot improve the performance of the engine. In fact, the performance of the engine can be severely limited. This is because the heat produced during initial compression of the gaseous fuel/air mixture before the ignition process is commenced, is greater in a charged engine than in an uncharged engine, and the additional heat can cause the gaseous fuel/air mixture to explode rather than burn progressively, or worse still, the mixture can start burning from self-ignition in an uncontrolled manner before ignition is introduced, and before maximum compression is reached. Hence, the power output of the engine may be decreased. Therefore, it is usual for an engine operating on a gaseous fuel not to be turbocharged or supercharged, or if they are, to a lesser extent than diesel engines.